OLEDs are electroluminescent devices which have been intensively studied over the past two decades, and have become competitive with established solutions in illumination tasks requiring large-area light sources. Conventionally, OLEDS have consisted of one or more light-emitting organic layers (generally referred to as the light-emitting organic stack) sandwiched between a metallic cathode and a transparent anode deposited on a transparent substrate. Current flowing through the OLED causes the charges carriers (holes and electrons) to recombine in the emissive layer and emit photons. The light-emitting organic layer(s) may be polymeric, and such OLEDs are sometimes referred to as PLEDs. Alternatively or additionally, the light-emitting organic layer(s) may comprise a non-polymeric, small-molecule organic compound, and such OLEDs are sometimes referred to as SMOLEDs. The visible light emitted from an organic layer or layer stack may be white light, or a narrower range of wavelengths in the visible region, depending on the organic molecule and the intended application. Suitable light-emitting organic compounds include the commercially available Livilux™ materials (Merck). For most lighting applications, the output of an OLED light source should be independent of the emission wavelength over the visible spectrum, i.e. a white light is desired. Various approaches have been proposed for the emission of visible white light from an OLED light source, including single white-emitting layers and vertical or horizontal stacks of different organic layers each emitting in a different wavelength in the visible region (typically red-green-blue (RGB) stacks). Typically, the thickness of the OLED stack including the electrodes is not more than 1 μm, whereas the substrate thickness is up to and around 1 mm. Because the organic light-emitting layers are very sensitive to air and moisture, the OLED stack is encapsulated within a barrier. The majority of commercially available devices are currently fabricated on rigid glass substrates, although flexible plastic substrates are now being adopted, which are becoming increasingly important when utilized with roll-to-roll continuous manufacturing techniques. In addition to their use as light sources, OLEDs which are transparent (TOLEDs) have been proposed for integration into buildings and vehicles as smart-windows. Additionally, the control of electromagnetic radiation across a window may enable cost savings in terms of the heating and cooling of rooms. The integration of OLEDs and photovoltaic (PV) cells has also been proposed.
The manufacture and application of OLED light sources are now well known and a variety of architectures has been developed, for instance “bottom emission” and “top emission” designs. The bottom-emitting device comprises a transparent bottom substrate layer (traditionally glass) on which the OLED is fabricated, a transparent anode layer, a light-emitting organic layer, a reflective cathode and an encapsulating layer, in that order. The top-emitting device comprises a bottom substrate layer, a reflecting layer and an anode layer (or a reflecting anode layer), a light-emitting organic layer, a transparent cathode and an encapsulating layer, in that order. The OLEDs comprise at least one transparent electrode, for instance a transparent conductive oxide (TCO) such as indium tin oxide (ITO). ITO is commonly used in OLEDs because of its high transparency, thermal stability and conductivity. Conductive organic materials, alone or in combination with TCOs, are also now being investigated for use as transparent electrodes. One advantage of organic materials as electrodes is the ability to tune their refractive index either to match or intentionally mismatch the organic light-emitting layers.
With improvements in the efficiency of OLEDs, their utility for lighting applications has increased significantly. In order to achieve high power efficiencies of lighting devices with high colour quality and appropriate colour coordinates, electroluminescence has to be generated free of electrical and optical losses. Light extraction from a planar OLED design that consists of multiple thin organic and inorganic films is hampered by a combination of optical phenomena. Even if state-of-the-art OLED devices can achieve an internal quantum efficiency (IQE; photons generated per injected electron) approaching 100%, only about 50% of the photons generated will propagate into the substrate, and only around 20% of the photons generated will escape the OLED, the rest having been wave-guided and/or absorbed in the OLED stack and substrate. The resulting external quantum efficiency (EQE; photons emitted into air from the device per injected electron) from an optical point of view is therefore at most about 20% when using high efficiency (phosphorescent) materials. This value quickly drops by roughly a factor 4 when using less efficient (fluorescent) materials.
The term “wave-guiding” or “wave-guided” refers to the internal reflection of light within a layer and the refraction of light at layer boundaries, which occurs as a result of refractive index differences between adjacent layers, to which Snellius' law can be applied. Wave-guiding results in propagation of light parallel to the plane of the OLED. The refractive index of a glass substrate is around 1.5 whereas the light-emitting organic layers typically have refractive indices of about 1.7 to 1.9. Wave-guiding within the organic/inorganic thin film layer(s) and/or substrate of the stack means absorption losses become more significant. Due to high extinction coefficients of the functional materials of the various layers in the OLEDs, lateral wave-guiding in these functional layers has a length scale many orders of magnitude lower than in loss-free media, such as glass. With an efficiency target for OLED lighting towards 150 Im/W, the optical loss factors must be simultaneously reduced with the electrical losses and losses due to non-radiative recombination in the OLED stack. The improvement of light extraction from these devices is sometimes referred to as out-coupling. Current commercially available OLEDs have an efficiency of around 40 Im/W, although OLEDs with an efficiency of up to about 100 Im/W have been disclosed in the literature. This can be compared to about 15 Im/W for incandescent lighting and 60 to 100 Im/W for fluorescent lighting. In further contrast to incandescent lighting, fluorescent lighting, and inorganic LED-based lighting (for instance, AlGaInP devices and InGaN devices), an advantage of OLED-based lighting is that the OLED device itself may be the luminaire, rather than merely the light bulb in a luminaire, and so does not suffer from the fixture losses associated with such other light sources.
Thus, substrate wave-guiding can account for losses of up to about 30% of the total power emitted by a radiating dipole (depending on the design of the OLED), and a number of studies have addressed modification of the substrate in order to improve light extraction and increase the efficiency of the OLED. For instance, it is known to modify the topography of the external surface of the substrate by introducing periodic micro-structures of sufficient size with respect to the active area of the OLED, which increases the direct emission of light from the substrate, and such structures include micro-spheres, micro-pyramids and other micro-lens arrays (see, for instance, Greiner et al., Jap. J. Appl. Phys., 2007, 46 (7A), p4125; and Yang et al., Appl. Phys. Lett. 2010, 97, p223303). It is also known to dispose a volume-scattering layer containing light-scattering particles such as TiO2 in an organic layer on the external surface of the OLED (Greiner et al., supra). Both approaches extract light from the substrate by redirecting the light via multiple reflections and require a high reflectance of the reflecting electrode to be effective. Further approaches involve the application of surface texturation or photonic crystals to the substrate (see Tyan et al., SID Digest, 2008, 39(1), p933). Such external extraction structures (EESs) have been shown to improve light output by as much as 60%. Internal extraction structures (IESs) have also been investigated in order to improve light extraction from wave-guided light within the organic and/or electrode layers of the OLED stack and the boundaries thereof, which can present different challenges because of the fragility of the organic stack and the potential for mechanical or chemical instability thereof. Studies have also been conducted which combine internal and external extraction structures in OLEDs (Vandersteegen et al., Light-Emitting Diodes: Research, Manufacturing and Applications XI; Eds. K. P. Streubel & H. Jeon; Proc of SPIE Vol. 6486, 64860H, 2007).
A further consideration is that it can be desirable for the light output of OLED lighting utilities to be viewing angle-independent, also referred to as a Lambertian output distribution. Thus, it can be desirable for optical properties such as emission intensity or emission colour not to deteriorate as the viewing angle changes from 0° (i.e. the normal direction; forward emission) through to 90°. The applicant's co-pending international application no. PCT/GB2013/053276 addresses the problem of angular variations in colour and intensity and provides an out-coupling film which provides a more homogeneous emission colour and/or emission intensity over the entire viewing angle range. In other OLED lighting utilities it can be desirable to increase the directionality of light output (typically in the normal direction, which is typically the direction of maximum light output efficiency) without detriment to the total light output efficiency of the device.